The intensity of electric lights is commonly given as so many candlepower, i.e., so many times the intensity of a standard candle. Since an ordinary candle is not a sufficiently accurate standard, the unit of intensity has been defined in various ways. It was originally defined as the luminous intensity in a horizontal direction of a candle of specified size burning at a specified rate. Later the international candle was taken as a standard; not actually a candle, it is defined in terms of the luminous intensity of a specified array of carbon-filament lamps. In 1948 a new candle, about 1.9% smaller than the former unit, was adopted. It is defined as 1/60 of the intensity of one square centimeter of a black body radiator at the temperature at which platinum solidifies (2,046°K;). This unit is sometimes called the new international candle; the official name given to it by the International Commission of Illumination (CIE) is candela.
Other quantities of importance in photometry include luminous flux, surface brightness (for a diffuse rather than point source), and surface illumination. Luminous flux is the radiation given off in the visible range of wavelengths by a radiating source. It is measured in lumens, one lumen being equal to the luminous flux per unit solid angle (steradian) emitted by a unit candle. Surface brightness is measured in lamberts, one lambert being equal to an average intensity of 1/π candle per square centimeter of a radiating surface. The intensity of illumination, also called illuminance, is a measure of the degree to which a surface is illuminated and is thus distinguished from the intensity of the light source. Illumination is given in footcandles, i.e., so many times the illumination given by a standard candle at 1 ft. Another unit of illumination is the lux, one lux being equal to one lumen incident per square meter of illuminated surface. One lux equals 0.0929 footcandle.
Instruments used for the measurement of light intensity, called photometers, make possible a comparison between an unknown intensity and a standard or known intensity. They are based on the inverse-square law, which states that as a light source is moved away from a surface it illuminates, the illumination decreases in an amount inversely proportional to the square of the distance. Thus the illumination of a surface by a source of light 2 ft away is 1/4 of the illumination at 1 ft from the source. Conversely, for two light sources, one at 1 ft from a surface and the other at 2 ft, to give the same illumination to the surface, it would be necessary for the source at 2 ft to have an intensity 4 times that of the source at 1 ft.
A photometer measures relative rather than absolute intensity. The Bunsen photometer (named for R. W. Bunsen) determines the light intensity of a source by comparison with a known, or standard, intensity. The two light sources (one of known, one of unknown intensity) are placed on opposite sides of the surface (a disk of paper) to be illuminated. In the center of this surface is a grease spot that, when illuminated equally from both sides, will appear neither lighter nor darker than the paper but will become almost invisible. Using the inverse-square law, the intensity of the unknown light source can be easily determined when the relative distances at which the two sources produce equal illumination are known. The Rumford photometer (named for Count Rumford), or shadow photometer, compares intensities of light sources by the density of the shadows produced. In the Lummer-Brodhun photometer, an opaque screen is placed between the two sources, and a comparison is made possible by an ingenious arrangement of prisms.